9.3.3 Position Packet Structure
The position packet provides an echo of the NMEA sentence received from an external time source as well as the Pulse
Per Second status and a time stamp representing when the position packet was assembled.
The position packet is a 554 byte UDP packet received on port 8308. The structure of the position packet is given in
Table
9-3 below
.
Note: You may notice the timestamps in the position packets are occasionally out of order with respect to the data pack-
ets. This is normal as the delivery of data packets is the sensor's highest priority, and a position packet may get deferred
momentarily in favor of transmitting a data packet. The ratio of data packets to position packets is about 14 to 1. See
NMEA
Message Formats on page 46
for details on the GPRMC message.
Number of Bytes
Byte Offset from
beginning of
packet (hex)
Description
42 0x0000 UDP header
198 0x002A unused (null bytes)
4 0x00F0 Timestamp (µs)
1 0x00F4 Pulse Per Second status
3 0x00F5 unused
variable length 0x00F8 NMEA GPRMC sentence
Table 9-3 Position Packet Structure Field Offsets
Payload length is 512 bytes. The GPRMC sentence is terminated with CR/LF and padded to end of payload with null
bytes.
Value Description
0 No PPS detected
1 Synchronizing to PPS
2 PPS Locked
3 Error
Table 9-4 PPS Status Byte Values
A position packet is shown in the figure below. Packet offsets are on the left, raw data bytes in hex are in the center, and the
ASCII interpretation is on the right.
60 VLP-16 User Manual
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